I just read the book Meet Addy, and all (I think) of the follow-on books about Addy, from the American Girl collection. If they were collected under one cover, it would make a good 5th grade size of book. Published separately, of course, they net the publisher more money!
Even though that sounds pretty cynical (it was), the story is a good one. It's a happily-ever-after style story where everything works out in the end, but it does a decent job of portraying life and its challenges in the Civil War and post-war era for former black slaves. I highly recommend getting it from the library! Because they are short books, they shouldn't be too daunting for even the most reluctant readers.